Organic Chemistry I

The side-by-side orbital overlapping forms
the π (pi) bond.
1.6 Valence Bond Theory and Hybridization | 37

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Figure 1.6o Side-by-side overlap of p orbitals leading to pi (π) bond
So now we understand that the C=C double bond contains two different bonds:
σ
(sigma) bond from sp
2
–sp
2
orbital
overlapping and π (pi) bond from 2p–2p overlapping. Because of the π bond, the overall shape of the whole C
2
H
4
molecule is co-planar.
The other sp
2
hybrid orbitals on each carbon atom overlap with 1s orbital of H atoms and give total four C-H
σ
(sigma)
bonds.
Figure 1.6p Sigma (
σ
) bond
framework of C2H4
Ethyne (C
2
H
2
)
Figure 1.6q Ethyne
hybridization
Ethyne C
2
H
2
(common name is acetylene) has a C
≡
C triple bond. Generally, triple bonds involve one
σ
sigma bond and
two π (pi) bonds. Both carbon atom is in sp hybridization and in linear shape. With sp hybridization, each carbon has
two sp hybrid orbitals and two unhybridized 2p orbitals. Each carbon uses one sp hybrid orbital to overlap head-to-head
and gives the C-C the
σ
sigma bond, meanwhile the 2p orbitals overlap side-by-side to give two π bonds as shown in the
diagram below. The other sp orbitals are used for overlapping with 1s of hydrogen atoms to form C-H 
σ
bonds.
Figure 1.6r Orbital hybridization diagram of valence electrons in Ethyne
38 | 1.6 Valence Bond Theory and Hybridization

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Figure 1.6s Sigma (
σ
) bond framework of Ethyne and two pi (π) binds of Ethyne